Diesels take 10 - 12 miles to warm up fully in the current weather, and while they're arriving at a destination less than 10 miles away, you might be lucky to crack 40mpg. I could get 30-32mpg in my R over the same journey. Petrol mpg isn't critical to oil/engine temp - there is a benefit, but the difference between a hot and cold petrol for mpg is much smaller than for a diesel.